Retroreflective materials and surfaces are widely used to promote safety in traffic signs and high visibility clothing. Utilizing the property of retroreflective materials to reflect incident light generally back to the direction from whence it came, this effect results in highly visible features when viewed from objects with light sources, such as the headlights of cars. Retroreflection (sometimes called retroflection) is typically used on road surfaces, road signs, vehicles, and clothing. For example, when the headlights of an automobile illuminate a retroreflective surface, the reflected light is generally directed back towards the car and driver, rather than in all directions as with diffuse reflection or scattering.
The contrast between the return signal of a retroreflective surface and an adjacent area which does not retroreflect is high and readily detected. Therefore, scanning a beam across the edge of an object which itself does not retroreflect, in juxtaposition with a retroreflective surface either adjacent or in the background, gives a strongly differentiated signal which allows the location of the edge to be determined with high accuracy. As described herein, this effect is used to accurately locate the position of edges and other specific features of objects (such as a notch) which are critical in determining the object's orientation. The use of fiducials on work pieces or test articles can be used for a variety of metrology purposes to be described. In one embodiment, the present invention accurately determines the location of wafer edges in semiconductor processing, as well as, for example, the location of the wafer center and the orientation of the wafer notch. The knowledge of these parameters is important for many processes.
In many industrial processes, as for example in semiconductor wafer processing, a beam of energy, such as a laser, electron, or ion beam, is scanned over a work piece in order to deposit energy of a particular character. Such an energy beam can also be called a process beam. For example, in semiconductor wafer processing, a beam of actinic radiation, such as a ultra-violet, deep ultraviolet, or extreme ultra-violet radiation (UV, DUV or EUV), or a beam of electrons or ions, may be scanned across a substrate for the purpose of energy deposition. In other cases, a visible or infra-red (IR) beam may be scanned across the substrate. The energy deposited by the beam may be controlled point by point as the beam scans, in order to deposit a particular energy pattern, or a so-called dose map (for example, in units of milli-Joules per square centimeter—mJ/cm2). The deposited energy may be imparted for many reasons, for example to chemically modify the surface or heat the surface for many purposes including annealing or promoting chemical reactions.
Many methods of scanning a process beam over a work piece are known in the art. For example, mirrors or prisms affixed to mechanically scanning or rotating stages may be used to sweep the beam across the substrate. Specifically, a two-axis galvanometer stage with affixed beam deflection mirrors may be used to scan a beam across a substrate. By slowly scanning one mirror (to be named a Y axis mirror in this example), whilst quickly scanning a second mirror orthogonal to the first (to be named an X axis mirror in this example), the entire substrate may be scanned and treated. Other beam deflection methods are also well known, including electrostatic or magnetic deflectors for charge particle beams, electro-optic or acousto-optic beam deflectors for light beams, and micro-mechanical beam deflectors and steering devices for light beams.
In many such devices for actuating beam steering and scanning, the desired amount of beam deflection is controlled by an input signal such as an analog or digital voltage. For scanning mirror galvanometer devices, for example, the angle of beam deflection from the mirror is controlled by an input voltage. The positional accuracy of energy dose imparted to the work piece surface by the beam is controlled by two factors as the beam scans: (1) the ability to place the beam at the correct position on the work piece at the correct time, and (2) the ability to actuate or control the intensity or power of the process beam to the correct level for that beam position, commensurate with the desired dose map. Many methods of controlling process beam intensity or power as a function of time are well known in the art. For example, for light beams, acousto-optic or electro-optic shutters may be employed to precisely control beam energy, synchronized with beam deflection.
However, a problem exists when using an input signal to accurately control the position of a scanning beam on a work piece. For example, an input voltage signal which is intended to deflect a process beam to a particular portion on the work piece may become distorted by signal amplifiers and cause the beam to impinge on the work piece with a positional error. In addition, the deflection voltage signal, when received by the beam deflection servo electronics, may not deflect the beam to the desired angle due to non-linearity or drift of the deflection electronics and/or mechanics. In addition, the beam, as deflected to a particular angle, may not land on the work piece at the desired location due to distortion imparted by intervening optical components such as mirrors or lenses. In addition, the beam, when landing on the work piece at a particular location, may not be correctly registered to the work piece due the work piece having been shifted from its desired location or orientated improperly. In addition, the beam, when landing on the work piece at a particular location, may not have the desired X and Y axis scale factors due to the work piece being placed at the incorrect position along the optical path of the beam deflection system or other factors.
When performing scanning beam energy deposition, in some cases a uniform energy deposition may be desired. In other cases an identical energy deposition map or pattern may be desired for all work pieces. In still other cases it may be desired to change the deposition map for each work piece to follow some industrial process. Finally, it may be desired to change the deposition map for each work piece by pre-correcting the deposition map for known systematic errors of a subsequent or antecedent industrial process step. Generally, however, it is critical to achieve accurate registration of the imparted energy map or pattern on the work piece, and to accurately control the amount of energy deposited at each location on the work piece. For example, for the case of semiconductor manufacturing, it may be desired to control the accuracy of beam position on the substrate to better than 1 mm, 100 microns, 10 microns or even less.
For many applications, it may be difficult to accurately ascertain the position of the process beam with respect to the work piece surface, for example due to the nature of the environment or of the beam itself. In some cases, for example, the work piece may be processed in a chamber which is inconvenient or hostile to typical kinds of sensors such as industrial cameras. In other cases, the nature of the process beam itself, for example a high power UV laser beam, may return very little useful signal to a camera as it traverses the work piece. In other cases, exposure of the work piece to the energy beam, for example, for the purpose of determining the work piece position, may impart energy to the surface which may not be desired.
In order to avoid these limitations, a probe beam may be used in conjunction with the process beam for the purpose of probing work piece boundaries, and other fiducials as taught herein, for the purpose of determining the work piece location and orientation without causing undesired energy deposition. In this case it is important that the probe beam not have the same effect on the work piece as desired by the process beam, or at least have a very small effect, so that the action of probing the work piece with the probe beam will not compromise the desired energy deposition accuracy of the process beam. For example, in many cases when using a high power UV or DUV process beam, a low power red or infrared (IR) laser, for example a helium-ion laser or diode laser, can safely be used as a probe beam.
In order for the probe beam to be able to accurately determine the position and orientation of the work piece with respect to the process beam, the probe beam must in effect act as an accurate surrogate for the process beam. In other words, as the process beam traverses the work piece in response to a deflection input signal, the probe beam should be able to also traverse the work piece at exactly the same position as the process beam in response the same deflection input signal, or at least with a known and stable offset with respect to the process beam. Described herein are means for achieving this requirement.
In a further embodiment of the present invention, an exposure field can be mapped by the probe beam with high accuracy to determine scale linearity factors. The factors are measured by incorporating fiducials on work pieces or test articles containing retroreflective features at precisely known locations within the processing plane. These measured factors can then be converted into beam deflection corrections which can be incorporated by the beam positioning means to reduce beam positioning error. These corrections can be implemented, for example, by appropriate electronics, controls and/or software.
The comparison of intended beam positions with probed beam positions results in a mapping of scan errors for each position probed. The information gathered by this process allows accurate determination of the coordinate reference frame of the probe beam, which can then be used to provide accurate positioning of either probe or process beam on the process surface of interest (such as the surface of a semiconductor wafer).
The relative motion of the probe or process beam over the work piece can be achieved in a number of ways. For example, the beam may be scanned using galvanometers, rotating prisms or deflections using acousto-optic or electro-optic devices. Alternatively, the beam may be stationary and the object may be moved, for example with a stage. What is important is a relative motion of the beam with respect to the object and fiducials affixed thereto, such as edges or notches.
Methods and apparatuses for performing measurements capable of determining the accurate position and orientation of a substrate with respect to a process beam are described herein. Further, methods and apparatuses for performing measurements capable of determining, with high accuracy, distortions of the coordinate reference frame of the beam deflection system with respect to the objects to be processed are also disclosed. Still further, methods and apparatuses wherein said distortion maps can be used to correct the beam deflection system by use of hardware and software means in order to substantially improve the linearity of beam deflection and scanning across the object, thereby achieving superior process beam energy deposition accuracy on the work piece, are also disclosed.